Typo in Drush git installation instructions

(in the "new" branch)

Line 58 of README.txt:

2a. To install Drush from source, add one of these entries to your manifests:

       include drush::git:drush

   or

       class {'drush::git:drush':
         git_branch => '8.x-6.x',
         update     => true,
       }

In both examples, the class name is missing a second colon between 'git' and the second 'drush'. It is 'drush::git:drush', but it should be 'drush::git::drush'.

This causes the following error in puppet:

Puppet::Parser::AST::Resource failed with error ArgumentError: Could not find declared class drush::git:drush